A current semiconductor device includes, as a component element, a MOS device such as MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) or the like. With a gate oxide film of a silicon MOS device, a thickness reduction is accelerated with the requirements of integration and high performance of the circuit. It is expected that the thickness comes to 1 nm or less from 2010 onward.
The current silicon oxidizing method rely on mainly dry O2 oxidation and wet O2 oxidation, by which, however, device formation having long TDDB (Time-Dependent Dielectric Breakdown) life and hot carrier resistance is not carried out easily. Further, as a thickness reduction of an oxide film progresses, there arose problems of quality degradation due to existence of a silicon sub-oxide layer (structural transition layer) produced in the vicinity of an interface between an Si substrate and an oxide film and thermal degradation due to high-temperature heat treatment (1,000° C., for example). Furthermore, with a thin film of 10 nm or less thickness, it is known that existence of a transition layer (incomplete oxidation layer) of an interface between a silicon oxide film and a silicon causes start of degradation of the characteristics of withstand voltage, leak current, and the like.
In order to solve those problems, it is studied oxidation with oxygen gas containing ozone of several % concentration by using an ozonizer. When colliding with the Si surface, ozone is dissociated to generate oxygen radicals having higher reactivity than that of oxygen molecules, allowing achievement of a high-quality oxide film with less sub-oxide at low temperature (Journal of Applied Physics 87, 8184 (2000), for example). Moreover, another method copes with the problems by carrying out post annealing in a nitrogen-gas atmosphere or the like to reduce electron trap site and hole trap site in the film.
On the other hand, with a tunnel oxide film for flush memory, a high electric field (>6 MV/cm) is applied to the thin film of 10 nm or less to carry out writing/reading of information (electronic loading/unloading), thus requiring high withstand voltage of the oxide film and high reliability thereof at voltage application (economic stability). Imperfection (existence of a defect and a sub-oxide layer) of the oxide film also becomes a cause of lowering the reliability.
A silicon oxide skin for TFT (Thin Film Transistor) has a film formed on a glass substrate, requiring processing at 400° C. or less. Due to relatively large required film thickness (50–100 nm), it adopts currently principally a chemical vapor deposition (CVD) method. In addition, silicon oxide films on silicon are used as insulating films (device separating film) in various spots of silicon devices. Though they are prepared principally by the CVD process, it is needed achievement of high quality and enhancement in reliability. Preparation of some insulating films is carried out at the final stage of process, and it is preferable to lower the preparation temperature as much as possible so as not to cause damage to device parts already prepared.
The concentration of ozone obtained by passing high-purity oxygen gas through an ozonizer is low or 5–10%. And ozone is dissociative gas, which has the property of causing intense dissociation by mutual collision as the temperature becomes high. As a result, the ozone concentration is lowered before ozone reaches a sample to be oxidized, and it becomes uneven between upstream and downstream ends of a heated wafer. With this, oxidizing force of ozone cannot sufficiently be exploited, raising problems such as uneven thickness of an oxide film and the like.
Moreover, some materials (metal, particularly) used in an ozone supply system and an ozone oxidation-processing system are apt to reaction and corrosion with ozone. When adopting such materials for a gas supply line, a reaction product (metallic oxide having lower vapor pressure, particularly) is supplied, together with ozone, to a sample to be oxidized, becoming a cause of lowering the quality of an oxide film.
Incidentally, plasma-oxygen oxidation and ultraviolet-light irradiation oxidation are currently known as a process for forming a good-quality oxide film from (poly)silicon by low-temperature (oxidation or reforming). In the former, plasma is generated in the atmosphere containing oxygen molecules to generate active oxidation seeds having high kinetic energy (>10 eV ), such as oxygen-atom (molecule) ion and neutral oxygen atom (molecule), which are used as oxidation (diffusion) seeds to thereby realize a reduction in oxidation temperature. In the latter, ultraviolet light (excimer UV lamp, for example) is introduced into the oxygen-gas atmosphere to generate oxygen radicals, which are used as oxidation seeds.
However, the former raises a problem of damage to a film by ion oxidation seeds having high energy. On the other hand, the latter raises a problem that the absolute generation amount of radicals (free radicals) is restricted by the photon density of ultraviolet light. Moreover, both raise a problem that the high reactivity between generated active seeds (ion or radical) prevents the active seeds from being effectively led to a substrate to be processed, or requires a contrivance for system layout to lead them thereto.
Further, in recent years, with a polysilicon oxide film for TFT, which is formed by CVD, there arise problems of imperfection of the oxide film, such as greater defects of a SiO2/Si interface as compared with a thermal oxide film, lower withstand voltage due to film composition without perfect SiO2, and the like. As a result, oxidation using the radicals as oxidation seeds is often carried out before or during CVD process for the purpose of reforming of the interface. By way of example, it is reported that when processing is carried out in the oxygen-radical atmosphere during CVD process and after film deposition of about 6 nm thickness, oxygen atoms reach the interface to achieve film reforming (Chung Yi et al., J. Vac. Sci. Technol. B 19, 2067 (2001), for example).
However, this method requires a plurality of processes of oxidation processing. This becomes a cause of lowering the throughput.